1. Field of the Invention
The present invention relates to electrical circuits. More specifically, the present invention relates to electrical circuits having a differential structure that uses complementary devices for processing single-ended signals.
2. Background Information
A circuit may be designed to have either single-ended or differential structure, both in the analogue and digital circuit application. The single-ended structure is the structure of a circuit having paired terminals, where only one of the terminals is connected either input or output terminal. In comparison with the single-ended structure, double-ended structure is known in the art. The double-ended structure is the structure of a circuit having paired terminals, where both of the paired terminals are connected to a single input or output terminal. A differential pair structure is well known in the art as an example of the double-ended structure. Differential pair structure is the structure of a circuit having double-ended structure where signal is differentially inputted and processed to and in the circuit.
A single-ended structure consumes relatively low power and produces relatively low noise. However, when a single-ended structure circuit is operating in class A or AB mode, the circuit may suffer from clamping of signal, which results in harmonic component problem.
On the other hand, differential pair circuit is not vulnerable to the interference problem. The reason is that AC current, which is not provided from power source circulates only through the circuit. Since differential pair circuit is not susceptible to the interference problem, it is advantageously used in integrating a lot of system blocks into one chip. Further, differential pair circuit is used in mixers which generally requires high level of isolation among radio frequency, local oscillation, and intermediate frequency signals.
Moreover, since differential pair circuit has symmetrical structure, even order distortion may be reduced. Because of the characteristic of reduced even order distortion, differential pair circuit is widely employed in amplifiers and mixers provided in the direct conversion receivers, which suffers from DC offset problem and, therefore, requires relatively high level of rejection characteristic of second order distortion.
There have been a lot of approaches for incorporating the advantageous characteristics of the single-ended and differential pair structures, in the analogue and digital circuit applications.
Especially in the RF circuit field, in order to use the combination of the single-ended and differential circuits, balun (balance unbalance) circuitry was necessarily used for converting single-ended signal to differential signal or differential signal to single-ended one. Balun circuitry may be made from either passive or active devices. However, when balun circuitry is made of passive devices, size of balun circuitry becomes large and, therefore, it is not easy to integrate balun circuitry in a chip. When balun circuitry is made of active devices, relatively large power consumption of the balun circuitry becomes problem especially in the RF circuit applications. Additionally, noise generated by active balun decreases signal to noise ratio.
In order to overcome this problem, it was suggested that differential pair circuitry is integrated in a chip while passive balun circuitry is provided on a PCB, thereby connecting the differential pair circuitry in the chip to the balun circuitry on the PCB. This method, however, is disadvantageous in that the passive balun circuitry has large size and manufacturing cost becomes high.
As another attempts to integrate the single-ended circuitry and differential pair circuitry in a single chip, integration of active balun circuitry in a chip has also been tried. This method, however, suffers from DC power consumption and added noise of the active balun circuitry.
In view of the above, there exists a need for a single-ended differential circuit that uses complimentary devices that overcomes the above-mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.